Olefins with a tertiary carbon atom which forms part of a double bond undergo selective addition with alcohols to form tertioalkyl ethers. Such addition is an exothermic process and is catalysed by acids in general. Such reactions are used in the synthesis of high octane number oxygen-containing products such as MTBE (Methyl Tertio Butyl Ether), ETBE (Ethyl Tertio Butyl Ether), TAiME (Tertio Amyl Ether), or ETAE (Ethyl Tertio Amyl Ether). TAME and ETAE are obtained from a mixture containing two isoamylenes, 2-methyl-1-butene and 2-methyl-2-butene (3-methyl-1-butene is only slightly reactive). The selectivity of such ether synthesis reactions can also be exploited to separate tertiary olefins from the hydrocarbons which contain them. Separation of certain olefins from each other, for example the separation of isobutene and 1-butene, is difficult using a conventional distillation process. In contrast, separation of a tertiary alkyl ether (which is selectively produced) from the hydrocarbon cut from which it is produced is generally easy. Once isolated, the ether can be decomposed again to form the starting tertiary olefin and the alcohol employed. This is an endothermic process, in the presence of a generally acid catalyst and at a temperature which is higher than that used for synthesis. The tertiary olefin produced can thus be of high purity, depending on the optimised conditions.
Compared with other methods for the production of high purity tertiary olefins, such as those using isomerisation reactions, in processes using reactive distillation (for example isobuteneibutene separation by transforming 1-butene into 2-butene), the scheme which incorporates synthesis then decomposition of the ether benefits from any infrastructure relating to the increasing importance of ethers in reformulated gasoline. Many refineries throughout the world have pure ether production plants, for example for the production of MTBE.
A large amount of pure ethers such as MTBE is already available on the international market. This means that the production of high purity tertiary olefin, for example isobutene, from ether, for example MTBE, can easily be carried out throughout the world, including locations remote from the refineries where such ethers are generally produced.
The exploitation of the selectivity of tertio alkyl ether decomposition reactions to the corresponding tertiary olefins has long been known, as shown, for example, in European patent application EP-A-0 068 785 (Sumitomo), and a variety of acidic solids have been described as catalysts for these reactions. Thus, French patent application FR-A-2 291 958 (Snamprogetti) describes the use of salts, oxides or complexes of tetravalent uranium, which can be supported on an alpha alumina, for example, with Lewis acidity. United States patent U.S. Pat. No. 4,656,016 (Snamprogetti) describes the use of silica modified by the introduction of boron into its framework and, optionally, by cations (H.sup.+, NH.sub.4.sup.+ or a metal cation). International patent application WO-A-91/01804 (EXXON) describes the use of clay (montmorillonite, kaolinite, attapulgite, bentonite . . .). Finally, U.S. Pat. No. 5,095,164 describes the use of ion exchange resins, for example sulphonated styrene-divinylbenzene resins (which are also generally used in tertio alkyl ether synthesis processes). Amberlyst 15 from Rohm & Haas or M-31 resin sold by Dow Chemical can be cited in this respect.
One of the major disadvantages of the resins cited above is that it is impossible to use them at high temperatures, more precisely above 120.degree. C. At high temperatures, such resins de-sulphonate and thus lose their activity and/or acidity. Further, ether decomposition reactions are endothermic; thus the higher the temperature, the further the thermodynamic equilibrium of the reaction is displaced towards production of the olefin. An operating temperature limited to 120.degree. C. results in low ether conversion which is also limited by the laws of thermodynamics.
U.S. Pat. No. 5,095,164 describes a process for the decomposition of tertio-alkyl ethers in the presence, for example, of macroporous sulphonated styrene-divinylbenzene resins using a distillation apparatus. The catalyst is placed at the bottom of a column which operates between 50.degree. C. and 100.degree. C., preferably between 60.degree. C. and 80.degree. C. The thermodynamic equilibrium of the decomposition reaction, which is poorly positioned because of the low operating temperature, is displaced by elimination of the reaction products (tertiary olefin and corresponding alcohol) by distillation. However, such a process has problems in product purification. In particular, it uses large quantities of water to recover the alcohol. Further, the unconverted ether is recovered from the bottom of the column with non negligible quantities of alcohol. It must therefore be purified before recycling to the process.
Other catalytic solids, for example those based on alumina, silica or silica-alumina, require the addition of water to improve alcohol recovery, and avoid the secondary reaction of the formation of the corresponding dialkyl ether, which in the case of methanol is:
2MeOH.rarw. .fwdarw.Me--O--Me (=DME =DiMethyl Ether)+H.sub.2 O.
This has been described, for example, in United Kingdom patent application GB-A-1 165 479 (Shell) and in EP-A-0 589 557 (Sumitomo). However, the presence of water reduces the activity of the catalyst, by reducing its acidity (see, in particular, GB-A-1 165 479) and may require operating at a higher temperature, which can reduce the lifetime of the catalyst. Further, the presence of water induces a supplemental secondary reaction: the water reacts with the tertiary olefin produced to form an alcohol, such as in the case of the decomposition of MTBE (or ETBE): isobutene+H.sub.2 O.fwdarw.TBA (tertio Butyl Alcohol or 2-methyl-2-propanol). In that process, the yield of the desired tertiary olefin is observed to fall.
In general, the ether decomposition processes which are known to the skilled person use catalysts which have at least one of the following disadvantages: low activity, low selectivity, or low stability over time. Thus, for example, the process described by Exxon (application WO-A-91/01804), using a clay based catalyst, suggests a system which can regenerate the catalyst in situ. In addition, our application U.S. Pat. No. 5,171,920 can also be cited, which uses a catalyst based on silica modified by the addition of at least one element such as Li, Cs, Mg, Ca or La. Such solids are not very active due to a lack of acidity, and they have mediocre stability over time: the data given in Table 1 of Example 13 of that patent indicates that in 800 hours, the temperature must be increased by 50.degree. C. to maintain constant the level of ether conversion.